Lithophyses in the Obsidian of the Rocche Rosse, Lipari. 139 



of Vulcano ; but the specimens show in a specially striking manner 

 thepassage through various stages of lithophysal structure, from indis- 

 putable steam- vesicles with glassy walls to typical solid spherulites. 

 A full description is given of the formation of spherulites by a double 

 process — firstly, divergent growth from the margins of vesicles out- 

 wards, and, secondly, convergent growth inwards from the margins 

 towards the centres of the hollows, until in the smallest cases the 

 fibres from the opposite sides of the vesicle may meet in the centre, 

 producing a spherulite, which, but for the occurrence of intermediate 

 stages, might be supposed to have originated entirely by divergent 

 growth. The authors give details of the appearances presented by 

 intermediate stages of growth. 



The prevailing type of spherulite, both in Lipari and Yulcano, 

 shows in section a dusky fibrous central area, which may possess con- 

 centric as well as radial structure, surrounded by an irregular brown 

 cloudy zone of various width. The authors' studies lead them to 

 the conclusion that this type owes its characters to the dual mode of 

 growth, and therefore to the original presence of vesicles in the rock. 

 Commonly the process of infilling does not go so far as this ; on the 

 ends of the felspar fibres plates of tridymite are deposited, and this 

 seems to close the growth. It is clear that the lithophysal structure 

 of the Lipari obsidians was formed during the cooling of the mass, 

 and not by subsequent amygdaloidal infilling of vesicles. 



The authors discuss the effect of confined vapours on such rocks 

 as those forming the subject of the paper, noting that these vapours 

 may be kept at a high temperature for a considerable time, each 

 vesicle thus becoming a sphere of hydrothermal action ; so that if 

 the surrounding glass remains at a temperature little below its 

 fusion-point, crystallization will be promoted in it, and at the same 

 time the action of the vapour in the vesicle will produce reactions 

 on its walls. 



An Appendix, by Prof. Cole, treats of the lithophyses and hollow 

 spherulites of altered rocks. While admitting the presence of 

 true lithophyses in many of the Welsh lavas, he is not prepared to 

 abandon a former suggestion that the interspaces between successive 

 coats of the Conway lithophyses result from alteration of a formerly 

 solid mass. In the lavas of Esgair-felen and near the Wrekin he has 

 no doubt as to the production of " hollow spherulites " by ordinary 

 processes of decay. The typical Continental pyromerides are truly 

 spherulitic, as is much of the Wrekin lava. In the latter case and 

 that of the rocks of Bouley Bay it will be difficult to distinguish 

 between infilled primary and secondary cavities. 



May 25.— W. H. Hudleston, Esq., M.A., F.R.S., 

 President, in the Chair. 

 The following communications were read : — 

 1. "On Delpliinognathus conocephalus (Seeley) from the Middle 

 Karoo Beds, Cape Colony, preserved in the South- African Museum, 

 Capetown." By Prof. H. G. Seeley, F.R.S., F.G.S. 



